Opportunistic fungi, including Candida albicans, Aspergillus fumigatus, Cryptococcus neoformans, and Pneumocystis carinii, are a rapidly emerging class of microbial pathogens, which cause systemic fungal infection or “mycosis” in patients whose immune system is weakened. Candida spp. rank as the predominant genus of fungal pathogens, accounting for approx. 8% of all bloodstream infections in hospitals today. Alarmingly, the incidence of life-threatening C. albicans infections or “candidiasis” have risen sharply over the last two decades, and ironically, the single greatest contributing factor to the prevalence of mycosis in hospitals today is modern medicine itself. Standard medical practices such as organ transplantation, chemotherapy and radiation therapy, suppress the immune system and make patients highly susceptible to fungal infection. Modern diseases, most notoriously, AIDS, also contribute to this growing occurrence of fungal infection. In fact, Pneumocystis carinii infection is the number one cause of mortality for AIDS victims. Treatment of fungal infection is hampered by the lack of safe and effective antifungal drugs. Antimycotic compounds used today; namely polyenes (amphotericin B) and azole-based derivatives (fluconazole), are of limited efficacy due to the nonspecific toxicity of the former and emerging resistance to the latter. Resistance to fluconazole has increased dramatically throughout the decade particularly in Candida and Aspergillus spp.
Clearly, new antimycotic compounds must be developed to combat fungal infection and resistance. Part of the solution depends on the elucidation of novel antifungal drug targets (i.e. gene products whose functional inactivation results in cell death). The identification of gene products essential to cell viability in a broad spectrum of fungi, and absent in humans, could serve as novel antifungal drug targets to which rational drug screening can be then employed. From this starting point, drug screens can be developed to identify specific antifungal compounds that inactivate essential and fungal-specific genes, which mimic the validated effect of the gene disruption.
Of paramount importance to the antifungal drug discovery process is the genome sequencing projects recently completed for the bakers yeast Saccharomyces cerevisiae and under way in C. albicans. Although S. cerevisiae is not itself pathogenic, it is closely related taxonomically to opportunistic pathogens including C. albicans. Consequently, many of the genes identified and studied in S. cerevisiae facilitate identification and functional analysis of orthologous genes present in the wealth of sequence information provided by the Stanford C. albicans genome project (http://candida.stanford.edu). Such genomic sequencing efforts accelerate the isolation of C. albicans genes which potentially participate in essential cellular processes and which therefore could serve as novel antifungal drug targets.
However, gene discovery through genome sequence analysis alone does not validate either known or novel genes as drug targets. Ultimately, target validation needs to be achieved through experimental demonstration of the essentiality of the candidate drug target gene directly within the pathogen, since only a limited concordance exists between gene essentiality for a particular ortholog in different organisms. For example, in a literature search of 13 C. albicans essential genes validated by gene disruption, 7 genes (i.e. CaFKS1, CaHSP90, CaKRE6, CaPRS1, CaRAD6, CaSNF1, and CaEFT2) are not essential in S. cerevisiae. Therefore, although the null phenotype of a gene in one organism may, in some instances, hint at the function of the orthologous gene in pathogenic yeasts, such predictions can prove invalid after experimentation.
There thus remains a need to identify new essential genes in C. albicans and validate same as drug targets.
The present invention seeks to meet these and other needs.
The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety.